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“It was terrifying,” says the best-selling novelist, 65, from near Stroud in Gloucestershire. “My entire chest felt like a box of worms, and my heart was leaping from one rhythm to another. I thought I was about to have a heart attack.”

Katie’s heart had raced before when she exerted herself, but not like this. Her GP suspected atrial fibrillation (AF), an irregular and often abnormally fast heartbeat – the risk of which increases with age – and sent her to a specialist who fitted her with a heart monitor, as they need to record the heart’s rhythm during...